On high speed rotary presses such as are used for newspaper printing, the type impression has long been carried on curved metal plates attached to a printing cylinder. Earlier rotary presses used lead plates that were cast from mats which had been mechanically impressed from metal type. More recently, thinner metal offset printing plates have been used, produced by photochemical processes that eliminate the need for using metal in the composing room.
With the earlier cast plates, the plate was inked and the ink was transferred directly from the plate to the web. Presses using the more recent offset printing plates are necessarily more complicated because water as well as ink must be applied to the offset plate, and the press must include a blanket roller to which the ink is transferred from the offset plate and which in turn transfers it to the web. The nature of the offset plate is such that it holds water in its areas which are not intended to produce an ink impression, and the water repels the oily ink, which therefore settles only on the impression areas. The blanket roller has a surface which accepts the ink but rejects the water, and therefore it transfers only ink to the web.
Photopolymer flexible plates for rotary presses have been known for many years. Such a plate can be prepared by photochemical processes, and therefore it offers essentially the same preparation advantages as a metal offset plate. It possesses the further very important advantage that it need not be wetted with water, and therefore ink can be transferred directly from it to the web. This not only simplifies the press by eliminating the need for a blanket roller but has the further very important advantage of permitting the use of water-based inks which dry more quickly and thoroughly than oil-based inks.
Heretofore, however, the quality of printing obtainable with flexible plates has been substantially inferior to that obtained with metal plates, and therefore--with possible minor exceptions--flexible plates have not been used for newspaper printing.
In U.S. Pat. No. 3,858,511 and No. 3,896,729, the inferior quality of printing obtained with flexible plates is attributed to the problem of securely retaining such plates on the printing cylinder. Although satisfactory solutions have been found for that particular problem, the quality of printing heretofore obtained with flexible plates has still not been satisfactory for newspapers. It is well known that if flexible plates could produce printing of substantially the quality obtained with metal plates, there would be a prompt and wide-spread adoption of flexible plate printing by newspapers.
A basic cause of poor quality printing with flexible plates has been that such plates had irregularities in thickness that gave rise to irregular movements of the cylinders comprising the printing mechanism. Such thickness irregularities were not encountered with metal plates, which could be made to very close tolerances.
Earlier flexible plates tended to have such large thickness irregularities that it can be doubted whether high quality printing could ever have been achieved with them, but more recently a thinner type of flexible plate has become available that has substantially smaller thickness deviations. Nevertheless, these improved flexible plates present problems with respect to the press on which they are used. These problems are perhaps not unique to presses using flexible plates--they can be encountered in presses intended for use with metal plates--but they tend to be somewhat aggravated with flexible plates, and therefore the following discussion is primarily concerned with flexible plate printing although for the most part it is also valid for printing with metal plates.
In general, the above-mentioned problems are centered upon the prevention of erratic cylinder movements. Even though the more recently available flexible plates have only very small thickness irregularities--on the order of 0.001 inch--they are not perfect, and the press must accommodate their irregularities.
One consequence of irregularities in the thickness of a printing plate is that they give rise to a type of erratic cylinder movement that can be characterized as "cylinder bounce". During rotation of the printing cylinder on which the flexible plate is mounted, the plate is carried alternately into opposing relationship with an anilox cylinder, by which ink is transferred to the plate from an inking cylinder, and an impression cylinder by which the moving web is maintained in engagement with the plate. In each case the thicker portions of the flexible plate are resiliently compressed between the printing cylinder and the opposing cylinder, and the plate reacts by imposing divergent forces upon those cylinders whereby each tends to be bowed or laterally displaced along its length. It will be apparent that the press structure must be designed to prevent these more or less rhythmically recurring forces from reinforcing one another and building up to a lateral oscillation of the cylinders--particularly the printing cylinder--that would cause smearing and blurring of the imprint made by the plate.
Presumably cylinder bouncing could be prevented if the cylinders could be locked into fixed rotational positions on a rigid press frame, but that is out of the question. To enable printing plates to be changed and web to be threaded through the printing couple, as well as for other reasons, the several cylinders must be movable for throw-out and thow-in relative to one another, that is, movement between spaced apart and operative positions. As a further complication, the printing cylinder should be adjustable as to its operative position relative to the impression cylinder, to accommodate webs of different thicknesses and compensate for irregularities in plate thickness, and the anilox cylinder should be similarly adjustable relative to the printing cylinder. After throw-out, each of these cylinders should return to exactly its adjusted operative position, and it should maintain that position all during press operation.
Provision for adjustment of operative position of the cylinders involves another complication that can give rise to another type of erratic cylinder movement. Heretofore in rotary presses the printing cylinder and the several cylinders that cooperated with it were driven for synchronized rotation by meshing gears, one for each cylinder, and each such gear was coaxially locked to its cylinder. One consequence of this arrangement was that any adjustment of the operative position of a cylinder involved a change in the center-to-center distance between the drive gear for that cylinder and the meshing drive gears for its adjacent cylinders. The drive gears were made with undercut teeth, so that any such small change in the center-to-center distance between meshing gears would occasion no more than a small amount of rotational play or backlash between them. Nevertheless, such meshing gears could rotate with substantially complete absence of backlash only when their center-to-center distance happened to be adjusted to the minimum, and consequently there was almost always some small rotational play between cooperating cylinders, causing rotational irregularities that were detrimental to printing quality.
There are further complications to the problem of providing for adjustment of the operative position of each cylinder, and for throw-out and throw-in, while preventing cylinder bouncing and erratic cylinder rotation. The printing cylinder must be adjustable relative to the impression cylinder, and the anilox cylinder must be adjustable relative to the printing cylinder without affecting the adjustment of the printing cylinder. Both of these adjustments should be repeatable in the sense that it should be possible to bring a cylinder back to a given position of adjustment not only after throw-out but also after it has been shifted to an entirely different adjustment.
It should also be possible to throw out the printing cylinder while the anilox and the inking cylinders move in unison with it, to throw out the anilox cylinder while the inking cylinder moves in unison with it and irrespective of the position of the printing cylinder, or to throw out the inking cylinder irrespective of the positions of the printing and anilox cylinders.
Although the several cylinders should be rigidly locked in their respective adjusted positions and securely confined against any lateral deflection when the press is in operation, the means by which they are so confined should not interfere with their throw-out, throw-in or adjustment and should not require special attention when the cylinders must be moved to their thrown-out relationships or back to their operative relationships.
It is perhaps obvious that the cylinders must be securely confined against end play when the press is running, but this requirement must be satisfied without any sacrifice of lateral stability, adjustability and repeatability.